Light-emitting device, illumination device, and vehicle headlamp

ABSTRACT

A headlamp includes a light-emitting section which emits fluorescence upon receiving excitation light from a laser element. Energy intensity distribution of the excitation light, which is received by the light-emitting section, is a top-hat distribution.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/744,238, filed Jan. 17, 2013 which claims priority to Japanese PatentApplication No. 2012-008479 filed Jan. 18, 2012, each of which is herebyincorporated by reference in the present disclosure in its entirety.

TECHNICAL FIELD

The present invention relates to a light-emitting device, anillumination device, and a vehicle headlamp, each of which utilizesfluorescent light that a phosphor generates upon irradiation withexcitation light.

BACKGROUND ART

In these years, studies on light-emitting devices are becoming active,which light-emitting devices each (i) use, as an excitation lightsource, a semiconductor light-emitting element such as a light-emittingdiode (LED) or a laser diode (LD) and (ii) use, as illumination light,fluorescent light that a light-emitting section containing a phosphorgenerates upon irradiation with excitation light emitted from theexcitation light source.

An example of such light-emitting devices is lighting equipment forvehicle, which is disclosed in Patent Literature 1. This lightingequipment for vehicle uses an LED module or an LD module as anexcitation light source, and generates white light by irradiating, withexcitation light, a phosphor in the form of a small dot approximately0.5 mm or less in diameter.

CITATION LIST Patent Literature

Patent Literature 1

Japanese Patent Application Publication, Tokukai, No. 2004-241142 A(Publication Date: Aug. 26, 2004)

SUMMARY OF INVENTION Technical Problem

However, the efficiency of conversion by the phosphor is approximately90% at the maximum, and excitation light that was not converted intofluorescent light turns into heat. Therefore, as excitation densityincreases, temperature of the phosphor rises and the efficiency ofconversion by the phosphor decreases.

Particularly, in a case where excitation is caused by excitation lightin the form of a Gaussian beam (for example, a laser beam), thefollowing problem arises. That is, the beam creates (i) a region whichis irradiated with a peak (top) of the beam and (ii) a region which isirradiated with an edge portion of the beam. This results in a reductionin conversion efficiency in the region which is irradiated with the peakof the beam and, accordingly, causes a reduction in the total conversionefficiency.

Such a problem cannot be solved by the invention of Patent Literature 1.

The present invention has been made in view of the above problem, and anobject of the present invention is to provide a light-emitting devicecapable of preventing a reduction in efficiency of conversion by aphosphor, which reduction would occur due to heat of excitation light.

Solution to Problem

In order to attain the above object, a light-emitting device inaccordance with an embodiment of the present invention includes alight-emitting section which emits fluorescence upon receivingexcitation light from an excitation light source, energy intensitydistribution of the excitation light, which is received by thelight-emitting section, being a top-hat distribution.

Advantageous Effects of Invention

As described above, a light-emitting device in accordance with thepresent invention includes a light-emitting section which emitsfluorescent light upon receiving excitation light from an excitationlight source, and is configured such that energy intensity distributionof the excitation light received by the light-emitting section is atop-hat distribution.

Therefore, it is possible to bring about the following effect. That is,the light-emitting section is not excited by excitation light that islocally high in energy intensity. Accordingly, it is possible to reducethe possibility of temperature quenching due to locally-hightemperature.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view schematically illustrating a configuration of a laserelement in accordance with an embodiment of the present invention.

FIG. 2 is a view illustrating types of spatial light intensitydistribution of excitation light.

FIG. 3 is a cross sectional view schematically illustrating aconfiguration of a headlamp in accordance with an embodiment of thepresent invention.

FIG. 4 is a view illustrating an example of shape of a laser beam spoton a laser beam irradiation surface of a light-emitting section.

(a) and (b) of FIG. 5 are views illustrating other examples of shapes ofa laser beam spot on a laser beam irradiation surface of alight-emitting section.

FIG. 6 is a view schematically illustrating a configuration of a laserelement in accordance with another embodiment of the present invention.

FIG. 7 is a view schematically illustrating a configuration of a laserelement in accordance with a further embodiment of the presentinvention.

FIG. 8 is a view schematically illustrating a configuration of a laserelement in accordance with still a further embodiment of the presentinvention.

FIG. 9 is a view schematically illustrating a configuration of a laserelement in accordance with still yet a further embodiment of the presentinvention.

DESCRIPTION OF EMBODIMENTS Embodiment 1

The following description discusses an embodiment of the presentinvention with reference to FIGS. 1 to 5.

Technical Idea of the Present Invention

First, with reference to FIG. 2, a technical idea of the presentinvention is described. FIG. 2 is a view illustrating types of spatiallight intensity distribution of excitation light.

In a light collection optical system, a spot size of collected light isdetermined by (i) an angle of divergence of an excitation light sourceand (ii) a size of the excitation light source. Therefore, a size of anexcitation light source is important in the light collection opticalsystem.

In a case where a semiconductor laser is used as an excitation lightsource, the spatial light intensity distribution of a laser beam emittedfrom the semiconductor laser is usually a Gaussian distribution (theleftmost beam profile in FIG. 2). According to the Gaussiandistribution, the energy intensity is different between (i) a regionirradiated with a peak (top) of the laser beam and (ii) a regionirradiated with an edge portion of the laser beam.

Another example of the spatial light intensity distribution is a top-hatdistribution (the beam profile in the middle shown in FIG. 2). Thetop-hat distribution is such that the energy intensity of excitationlight across a surface which is irradiated with the excitation light isalmost uniform.

Assume that the amount of light is the same and beam width is the samefor a Gaussian distribution and a top-hat distribution. In this case,the light intensity in a region irradiated with a peak of excitationlight in the Gaussian distribution may be five times as high as thelight intensity in the top-hat distribution.

Excitation light which was not converted into fluorescence becomes heat.Therefore, temperature of a phosphor rises as excitation densityincreases. When the temperature of the phosphor rises, the efficiency ofconversion by the phosphor decreases due to temperature quenching.Therefore, when irradiation is carried out with the excitation lighthaving a Gaussian distribution, the conversion efficiency in a regionirradiated with a peak of the excitation light decreases, andaccordingly the total conversion efficiency may decrease. As a result,the total luminous flux of obtained fluorescence may decrease (maybecome darker) than those obtained by irradiation with excitation lighthaving a top-hat distribution.

In order to solve the above problem, according to the present invention,a beam having a top-hat profile is used as excitation light with whichthe light-emitting section containing a phosphor is to be irradiated.This prevents a reduction in the efficiency of conversion by thephosphor due to heat of the excitation light.

Further, a Gaussian profile has the following problem. Since a Gaussianprofile begins and ends with gradual curves, a contrast between a partwhich is excited and a part which is not excited is poor, and an opticaldesign becomes complicated. That is, the edge portion of the beam cannotbe used optically.

This problem can be solved by irradiation with excitation light having atop-hat distribution.

Note that, in a case where a beam having a Gaussian distribution, whichbeam (i) has a peak intensity that is the same as the intensity of abeam having a top-hat distribution and (ii) has the same width as thebeam having the top-hat distribution (the rightmost beam profile in FIG.2), is compared with the beam having the top-hat distribution, theradiation flux of the beam having the Gaussian distribution is 60% ofthe beam having the top-hat distribution. Further, in this case,excitation light emitted from a part of a light source, which part isoutside a region corresponding to the necessary size of the lightsource, becomes stray light in a light collection optical system, andforms an unintended image. This necessitates an additional opticalpart(s) such as a slit for blocking the stray light. Therefore, it ispreferable to irradiate with excitation light having a top-hatdistribution.

<Configuration of Headlamp 1>

The following describes a headlamp (a vehicle headlamp) 1 as an exampleof a light-emitting device or an illumination device in accordance withone embodiment of the present invention. The headlamp 1 meets thestandards for light distribution characteristics of a driving headlamp(high beam) for automobiles. Note, however, that the illumination devicein accordance with the one embodiment of the present invention can be aheadlamp for passing (low beam). Further, the illumination device inaccordance with the one embodiment of the present invention can berealized as a headlamp for a vehicle or a moving object other than theautomobile (for example, person, ship, vessel, airplane, submarine,rocket, or the like), or can be realized as an illumination device forother applications. The examples of such an illumination device includea search light, a projector, a household illumination apparatus, acommercial illumination device, and an exterior illumination device.

FIG. 3 is a cross-sectional view schematically illustrating aconfiguration of a headlamp (vehicle headlamp) in accordance with theone embodiment of the present invention. As illustrated in FIG. 3, theheadlamp 1 includes laser elements (excitation light sources) 2, lenses(second optical members) 3, a light-emitting section 4, a parabolicmirror (a projection member, a reflector) 5, a metal base 7, and a fin8.

The headlamp 1 is configured to generate fluorescence by irradiating thelight-emitting section 4 with excitation light emitted from the laserelements 2. The fluorescence is used as illumination light. The energyintensity distribution of the excitation light emitted to thelight-emitting section 4 of the headlamp 1 is a top-hat distribution.

(Laser Element 2)

A laser element 2 is a light-emitting element, which functions as anexcitation light source which emits excitation light. An example of thelaser element 2 is a semiconductor laser. Strictly speaking, a laserchip 21 (see FIG. 1) included in the laser element 2 can be regarded asan excitation light source. The laser element 2 includes an optical rod23 (a first optical member) for converting the energy intensitydistribution of a laser beam into a top-hat distribution by internalmultiple reflection. The configuration of the laser element 2 isdescribed later in detail.

The headlamp 1 can include a plurality of laser elements 2. In thiscase, each of the plurality of laser elements 2 emits, by laseroscillation, a laser beam serving as excitation light. The number oflaser elements 2 can be only one, but the use of a plurality of laserelements 2 makes it easier to obtain a high-power laser beam.

Further, the laser element 2 can be one that has a light-emitting pointin a single chip or one that has a plurality of light-emitting points ina single chip. The wavelength of a laser beam from the laser element 2is for example 405 nm (blue-violet) or 450 nm (blue). Note, however,that the wavelength is not limited to those. It is only necessary toselect wavelength as appropriate depending on the types of phosphor tobe included in the light-emitting section 4.

The excitation light source of the headlamp 1 can be an LED. Note,however, that it is preferable to use a semiconductor laser as theexcitation light source, because the semiconductor laser couples lightwith the optical rod 23 more efficiently than the LED.

(Lens 3)

A lens 3 is a second optical member which directs, to the light-emittingsection 4, a laser beam emitted from the optical rod 23 of the laserelement 2. The lens 3 controls the spot size of a laser beam and directsthe laser beam to the light-emitting section 4.

By providing the lens 3, it is possible to cause an exit surface 23 b ofthe optical rod 23 and a spot of excitation light incident on thelight-emitting section 4 to be optically conjugate with each other. Thismakes it possible to easily control a spot size of the excitation lighton the light-emitting section 4.

By changing the design of a curved surface of the lens 3 or changing thedistance between the lens 3 and the light-emitting section 4, it ispossible to change a spot size of a laser beam on a laser beamirradiation surface 4 a of the light-emitting section 4.

The lens 3 is for example a convex lens. The lens 3 is provided so as tocorrespond to each laser element 2. The positions of the lens 3 and thelaser element 2 are determined such that the exit surface 23 b of theoptical rod 23 coincides with one focal point of the lens 3. Thepositions of the lens 3 and the light-emitting section 4 are determinedsuch that the laser beam irradiation surface 4 a of the light-emittingsection 4 coincides with the other focal point of the lens 3.

A laser beam that comes out of the optical rod 23 is a laser beam whichhas a spatial light intensity distribution in the form of a top-hatprofile, and the lens 3 directs this laser beam to the light-emittingsection 4 (described later).

The second optical member included in the headlamp 1 can be a concavemirror, instead of the lens 3. The second optical member is notparticularly limited, provided that the second optical member is capableof controlling a spot size of a laser beam.

(Light-Emitting Section 4)

The light-emitting section 4 is one that emits fluorescence uponreceiving a laser beam emitted from the laser element 2, and contains aphosphor(s) (fluorescent material(s)) which emits light upon receiving alaser beam. Specifically, the light-emitting section 4 is (i) one thatis obtained by dispersing phosphors in a sealing member or (ii) one thatis obtained by solidifying a phosphor. The light-emitting section 4converts a laser beam into fluorescence. Therefore, the light-emittingsection can be regarded as a wavelength conversion element.

The light-emitting section 4 is provided on the metal base 7 andsubstantially at a focal point of the parabolic mirror 5. Therefore,fluorescence emitted from the light-emitting section 4 is reflected by areflective curved surface of the parabolic mirror 5, thereby the path ofthe fluorescence is controlled. On the laser beam irradiation surface 4a of the light-emitting section 4, an antireflection structure forpreventing reflection of a laser beam can be provided.

Examples of the phosphor included in the light-emitting section 4include oxynitride phosphor (for example, sialon phosphor), nitridephosphor (for example, CASN (CaAlSiN₃), and III-V group compoundsemiconductor nanoparticle phosphor (for example, indium phosphide:InP). Note, however, that the phosphor included in the light-emittingsection 4 is not limited to those listed above, and can be otherphosphors.

Note that the law demands that illumination light emitted from aheadlamp must be white that has a chromaticity falling within a specificrange. Therefore, the light-emitting section 4 contains a phosphor(s)which is/are selected so as to obtain illumination light of specifiedwhite.

For example, when blue, green, and red phosphors are included in thelight-emitting section 4 and these phosphors are irradiated with a405-nm laser beam, white light is generated. Alternatively, it ispossible to obtain white light by providing a yellow phosphor (or greenand red phosphors) in the light-emitting section 4 and irradiating itwith a 450-nm laser beam (blue) (or a laser beam of so-called “close toblue” which has a peak wavelength falling within a wavelength range from440 nm to 490 nm).

The sealing member of the light-emitting section 4 is for example aglass material (inorganic glass, organic-inorganic hybrid glass) or aresin material such as a silicone resin. The glass material can below-melting glass. It is preferable that the sealing member is highlytransparent. In a case where a laser beam is high in power, it ispreferable that the glass material is highly resistant to heat.

(Parabolic Mirror 5)

The parabolic mirror 5 is an example of a projection member whichreflects fluorescence generated by the light-emitting section 4 andforms a bundle of rays (illumination light) which travels within apredetermined solid angle. The parabolic mirror 5 can be for example amember that has a metal film on its surface or can be a member made ofmetal.

A reflective surface of the parabolic mirror 5 includes at least a partof a partial curved surface, which is obtained by (i) rotating aparabola about its symmetric axis so as to form a curved surface(parabolic surface) and (ii) cutting the curved surface along a planeincluding the symmetric axis.

The laser element 2 is provided outside the parabolic mirror 5, and theparabolic mirror 5 has a window 6 which allows a light beam to transmitor pass therethrough. The window 6 can be (i) an opening or (ii) a partthat includes a transparent material capable of transmitting a laserbeam. For example, the window 6 can be a transparent plate provided witha filter that transmits a laser beam but reflects white light(fluorescence emitted from the light-emitting section 4). With thisconfiguration, it is possible to prevent fluorescence, which is emittedfrom the light-emitting section 4, from leaking out of the window 6.

One window 6 shared by a plurality of laser elements 2 can be provided,or a plurality of windows 6 corresponding to a respective plurality oflaser elements 2 can be provided.

Note that the parabolic mirror 5 can include a part that is not aparabola. Further, a reflecting mirror (reflector) of the light-emittingdevice in accordance with the one embodiment of the preset invention canbe (i) one that includes a parabolic mirror having a closed circularopening or (ii) one that includes a part of the parabolic mirror.Further, the reflecting mirror is not limited to the parabolic mirror,and can be an ellipsoidal mirror, a free-form surface mirror or amulti-mirror.

Further, a lens can be used as a projection member for projecting, in adesired direction, illumination light that includes fluorescence emittedfrom the light-emitting section 4. The lens is an optical system whichprojects fluorescence in a predetermined direction by transmitting andrefracting the fluorescence.

The light-emitting section 4, which is excited by a laser beam having anenergy intensity distribution in the form of a top-hat profile, emitsfluorescence having an energy intensity distribution in the form of atop-hat profile. In a case where such fluorescence is projected by theforegoing projection member (for example, the parabolic mirror 5),almost all of the luminous flux of the fluorescence emitted from thelight-emitting section 4 can be used. This makes it possible to achievea highly efficient projection system.

(Metal Base 7)

The metal base 7 is a supporting member in the form of a plate forsupporting the light-emitting section 4, and is made of metal (forexample, aluminum or copper). Therefore, the metal base 7 has a highthermal conductivity and is capable of efficiently dissipating heatgenerated by the light-emitting section 4.

Note that the supporting member which supports the light-emittingsection 4 is not limited to those made of metal, and can be a memberthat contains a material having a high thermal conductivity (siliconcarbide, aluminum nitride, or the like) other than metal. Note, however,that a surface of the metal base 7, which surface abuts thelight-emitting section 4, preferably functions as a reflection surface.With the configuration in which the surface is as a reflection surface,it is possible, after a laser beam that has entered the light-emittingsection 4 through the laser beam irradiation surface 4 a of thelight-emitting section 4 is converted into fluorescence, to cause thefluorescence to be reflected at the reflection surface and to direct thefluorescence toward the parabolic mirror 5. Alternatively, it ispossible to cause a laser beam that has entered the light-emittingsection 4 through the laser beam irradiation surface 4 a of thelight-emitting section 4 to be reflected by the reflection surface sothat the laser beam travels back to the inside of the light-emittingsection 4 and is converted into fluorescence.

(Fin 8)

The fin 8 functions as a cooling section (heat dissipation structure),which cools the metal base 7. The fin 8 has a plurality of heatdissipating plates, thereby the area in contact with air is increased sothat heat dissipation efficiency increases. The cooling section forcooling the metal base 7 is not limited provided that it has a coolingfunction (a function of dissipating heat), and can be of a heat-pipetype, water-cooling type or air-cooling type.

<Details of Laser Element 2>

FIG. 1 is a view schematically illustrating a configuration of the laserelement 2 in accordance with the present embodiment. As illustrated inFIG. 1, the laser element 2 includes a laser chip (excitation lightsource) 21, a sub-mount 22, an optical rod 23, an AR (Anti Reflection)coating film 24, a cap 25, a stem 26, and a lead terminal 27.

(Laser Chip 21 Etc.)

The laser chip 21 is a semiconductor laser element in the form of achip, which emits a laser beam serving as excitation light. Theconfiguration of the laser chip 21 (for example, a material of asemiconductor layer) is not limited in particular. The laser chip 21 isfixed on the sub-mount 22, and the sub mount 22 is fixed on the stem 26.

The sub-mount 22 functions as a heat sink for exhausting heat that wouldbe generated when the laser chip 21 operates. Therefore, the sub-mount22 can be made of aluminum nitride and/or SiC which have high thermalconductivity.

The stem 26 can be made of gold-plated iron, gold-plated copper, or thelike. It is preferable that the stem 26 is also made of a metal that hashigh thermal conductivity, in view of exhausting heat that would begenerated in the laser chip 21.

The laser chip 21 is connected with the lead terminal 27 via thin goldlines (not illustrated), and electricity is supplied from an externalpower supply via these lines.

(Optical Rod 23)

The optical rod 23 is an optical member (a first optical member) whichconverts the energy intensity distribution of a laser beam, which isemitted from the laser chip 21, into a top-hat distribution. The opticalrod 23 has (i) an entrance surface 23 a for receiving a laser beam and(ii) an exit surface 23 b through which the laser beam goes out. Theentrance surface 23 a is at one end of the optical rod 23, whereas theexit surface 23 b is at the other end of the optical rod 23.

The entrance surface 23 a lies in the vicinity of a light-emitting pointof the laser chip 21, and allows a laser beam emitted from thelight-emitting point to enter the optical rod 23 therethrough.

The laser beam emitted from the light-emitting point is light having aspatial light intensity distribution in the form of a Gaussian profile,but is converted into light having a spatial light intensitydistribution in the form of a top-hat profile by multiple reflectionwhile passing through the optical rod 23.

An exit end which includes the exit surface 23 b extends out of thelaser element 2 through the cap 25. The exit surface 23 b coincides withone focal point of the lens 3.

Therefore, the laser beam from the exit surface 23 b is efficientlydirected, by the lens 3, to the laser beam irradiation surface 4 a ofthe light-emitting section 4.

Further, the AR coating film (antireflection structure) 24 serving as anantireflection film is provided on the entrance surface 23 a. This makesit possible to prevent the laser beam from being reflected by theentrance surface 23 a, and thus possible to reduce losses of the laserbeam which would occur when the laser beam enters the optical rod 23.The AR coating film 24 can be provided on the exit surface 23 b. Notethat the AR coating film 24 is an example of the antireflectionstructure, and therefore some other antireflection structure (e.g. anantireflection structure including a moth-eye structure or the like) canbe provided on the entrance surface 23 a.

Not that the exit end, which includes the exit surface 23 b, does notnecessarily have to extend out of the laser element 2. The exit surface23 b can be positioned inside the cap 25, and a cap glass (window) whichtransmits a laser beam from the exit surface 23 b can be provided to thecap 25. Whether or not to cause the exit end which includes the exitsurface 23 b to extend out of the laser element 2 depends on the lengthof the optical rod 23, which length is necessary for converting a laserbeam into light having a spatial light intensity distribution in theform of a top-hat profile.

According to an example illustrated in FIG. 1, the length of the opticalrid 23 is 20 mm, and the diameter of the rod is 0.4 mm. Note, however,that the size of the optical rod 23 is not limited to the above.

FIG. 4 is a view illustrating an example of a shape of a laser beam spot41 on the laser beam irradiation surface 4 a of the light-emittingsection 4. The optical rod 23 is for example a rectangular prism, andthe exit surface 23 b is a quadrangle (for example, rectangle orsquare). The shape of the laser beam spot 41 corresponds to a shape ofthe exit surface 23 b. Therefore, it is possible to change the shape ofthe laser beam spot 41 to a desired shape by changing the shape of theexit surface 23 b.

With the configuration in which the exit surface 23 b of the optical rod23 is polygonal in shape, in a case where a plurality of optical rods 23or a plurality of laser beam spots 41 are to be arranged next to eachother, a space between the optical rods 23 or between the laser beamspots is reduced. As a result, it is possible to increase the density ofa laser beam, and possible to achieve efficient irradiation.

(a) and (b) of FIG. 5 are views illustrating other examples of the shapeof the laser beam spot 41 on the laser beam irradiation surface 4 a ofthe light-emitting section 4. As illustrated in (a) of FIG. 5, the shapeof the exit surface 23 b can be determined so that the laser beam spot41 is more complicated polygonal in shape. Alternatively, as illustratedin (b) of FIG. 5, the shape of the laser beam spot 41 can be the onethat includes a curved portion and an angular portion.

The illumination light emitted from the headlamp 1 forms an illuminationlight spot in a shape corresponding to the shape of the laser light spot41 on the light-emitting section 4.

The laser beam spots 41 illustrated in (a) and (b) of FIG. 5 each have ashape corresponding to the light distribution characteristic of aheadlamp for passing (low beam). By employing the laser beam spot 41having such a shape (that is, the exit surface 23 b having such ashape), it is possible to realize a light distribution pattern thatcorresponds to the light distribution characteristic of a headlamp forpassing (low beam).

That is, the optical rod 23 has the exit surface 23 b through which alaser beam goes out, and the shape of the exit surface 23 b correspondsto a desired light distribution pattern. This technical idea can also beapplied to a first optical member 29 and a multimode fiber 31 (describedlater).

Conventionally, a predetermined light distribution pattern has beenrealized by blocking part of illumination light with a mask(interception plate) or a mirror cut. However, this configuration causeslosses of the illumination light. By employing the exit surface 23 bhaving a shape corresponding to a desired light distributioncharacteristic, it is possible to realize the desired light distributioncharacteristic without causing losses of illumination light.

(Cap 25)

The cap 25 is a member for enclosing or sealing therein members such asthe laser chip 21. In particular, with the cap 25, the relativepositions of the laser chip 21 and the optical rod 23 are fixed. The cap25 can be hollow, or can be filled with a substance such as resin orlow-melting glass. Alternatively, the entire cap 25 can be made oflow-melting glass.

<Effects of Headlamp 1>

The headlamp 1 is configured such that the optical rod 23 is provided tothe laser element 2, which optical rod converts the spatial energyintensity distribution of a laser beam emitted from the laser chip 21into a top-hat distribution. With this configuration, the laser beamhaving a top-hat profile passes through the lens 3 and strikes the laserbeam irradiation surface 4 a of the light-emitting section 4.

Therefore, the laser beam irradiation surface 4 a is irradiated with alaser beam that has an almost uniform light intensity. As a result, itis possible to prevent the following problem. That is, the temperatureof a part of the laser beam irradiation surface 4 a rises due to thelaser beam, and the efficiency of conversion by the phosphor in thatpart decreases.

Further, the spatial light intensity distribution in the form of aGaussian profile has the following problem. That is, since the intensityof the laser beam on the laser beam irradiation surface 4 a graduallychanges from high to low as it spreads on the laser beam irradiationsurface 4 a, a contrast between a part which is excited by a laser beamand a part which is not excited by a laser beam is poor. In contrast, byirradiating the laser beam irradiation surface 4 a of the light-emittingsection 4 with a laser beam having a spatial light intensitydistribution in the form of a top-hat profile, it is possible toincrease a contrast between a part which is excited by a laser beam anda part which is not excited by a laser beam. This enables easy opticaldesign of the headlamp 1 and effective use of light beams.

The light-emitting section can be classified into two types, dependingon a relationship between (i) a direction from which a laser beam isincident on the light-emitting section 4 and (ii) a direction in whichfluorescence is emitted from the light-emitting section 4.

In the subject application, a light-emitting section that (i) receives,on its laser beam irradiation surface 4 a, a laser beam emitted from thelaser element 2 and (ii) emits fluorescence mainly to the opposite sideof the laser beam irradiation surface 4 a is referred to as “atransmissive light-emitting section”. According to a configurationincluding a transmissive light-emitting section, for example, thelight-emitting section 4 is provided at an opening of a substrate sothat (i) a laser beam incident on the light-emitting section 4 isconverted into fluorescence and (ii) the fluorescence is emitted mainlyto the side opposite to the side on which the laser element 2 isprovided.

In a case where the light-emitting section 4 is a transmissivelight-emitting section, a laser beam is scattered and thus a spatiallight intensity distribution is expanded inside the light-emittingsection 4. Therefore, it may be not necessary to provide an opticalmember for realizing a spatial light intensity distribution in the formof a top-hat profile. Note however that, even in this case, a spatiallight intensity distribution in the form of a Gaussian profile is kept.Therefore, the above case does not mean that a spatial light intensitydistribution in the form of a top-hat profile is realized.

On the other hand, a light-emitting section that (i) receives a laserbeam emitted from the laser element 2 and (ii) emits fluorescence mainlyto the side on which the laser element 2 is provided is referred to as a“reflective light-emitting section” in the subject application.According to the reflective light-emitting section 4 illustrated in FIG.3, the most intensive part of a distribution of fluorescence emittedfrom the light-emitting section 4 is included in a space where there isthe laser beam irradiation surface 4 a of the light-emitting section 4which receives a laser beam. The space is one of two spaces separated bya plane that includes a surface on which the light-emitting section 4 isprovided (i.e., a surface of the metal base 7). The reflectivelight-emitting section can be realized by leaving a space between theoptical rod 23 (a first optical member) and the light-emitting section4.

Note that, in a case where the light-emitting section 4 is a reflectivelight-emitting section, part of a laser beam may pass through thelight-emitting section 4. Even in this case, provided that fluorescenceis emitted mainly to the side on which the laser element 2 is provided,such a light-emitting section is referred to as a reflectivelight-emitting section.

As described earlier, according to a transmissive light-emittingsection, spatial light intensity distribution may be expanded due toscattering of a laser beam. Therefore, the light-emitting device inaccordance with the one embodiment of the present invention will betechnically valuable if realized as a light-emitting device including areflective light-emitting section. Note, however, that the presentinvention can be employed for the purpose of realizing a spatial lightintensity distribution in the form of a top-hat profile in alight-emitting device including a transmissive light-emitting section.

Further, in a case where the light-emitting section 4 is a transmissivelight-emitting section, it may be difficult to form the laser beam spot41 in a desired shape because a laser beam is diffused while passingthrough the light-emitting section 4. Therefore, in a case where theexit surface 23 b having a shape corresponding to a desired lightdistribution characteristic is to be employed, the present invention ispreferably realized as a light-emitting device including a reflectivelight-emitting section rather than a transmissive light-emittingsection.

Embodiment 2

The following description discusses another embodiment of the presentinvention with reference to FIG. 6. Note that members which are the sameas those in Embodiment 1 are assigned identical reference numerals, andtheir descriptions are omitted here.

According to the present embodiment, the headlamp 1 includes a laserelement 20 instead of the laser element 2. The other configurations arethe same as those of Embodiment 1.

<Configuration of Laser Element 20>

FIG. 6 is a view schematically illustrating a configuration of the laserelement 20 which is applicable to the headlamp 1. According to the laserelement 20, a side surface of the optical rod 23 is coated with analuminum coating 28.

Therefore, it is possible to prevent a laser beam from leaking out ofthe side surface of the optical rod 23 when the laser beam passesthrough the optical rod 23.

Note that a film with which the side surface of the optical rod 23 iscoated is not limited to an aluminum coating, and can be a reflectivefilm made of other highly-reflective material.

Embodiment 3

The following description discusses a further embodiment of the presentinvention with reference to FIG. 7. Note that members which are the sameas those in Embodiments 1 and 2 are assigned identical referencenumerals, and their descriptions are omitted here.

According to the present embodiment, the headlamp 1 includes a laserelement 40 instead of the laser element 2. The other configurations arethe same as those of Embodiments 1 and 2.

<Configuration of Laser Element 40>

FIG. 7 is a view schematically illustrating a configuration of the laserelement 40 applicable to the headlamp 1. The laser element 40 includes afirst optical member 29 instead of the optical rod 23. The first opticalmember 29 in accordance with the present embodiment is a hollow memberwhich has a plurality of inner surfaces 29 c serving as reflectivesurfaces.

The material and the configuration of the first optical member 29 in thepresent embodiment are not limited in particular. For example, theoptical member 29 is a hollow prism (for example, a rectangular prism)constituted by a plurality of mirror surfaces.

The length of the first optical member 29 is, for example, 20 mm, andthe length of one side of an exit end 29 b is, for example, 0.4 mm.Note, however, that the length of the first optical member 29 and thesize of the cross section of the first optical member 29 are not limitedto the above.

A laser beam enters the first optical member 29 through an entrancesurface 29 a, which is one end of the first optical member 29. The laserbeam which has entered the first optical member 29 travels while beingreflected by the inner surfaces 29 c of the first optical member 29, andgoes out through the exit surface 29 b, which is the other end of thefirst optical member 29.

The laser beam which has come out through the exit surface 29 b iscontrolled by a lens 3. Through the lens 3, the exit surface 29 b of thefirst optical member 29 and a spot of excitation light incident on alight-emitting section 4 are optically conjugate with each other.Accordingly, the laser beam which has come out through the exit surface29 b is efficiently directed to a laser beam irradiation surface 4 a ofthe light-emitting section 4 by the lens 3.

Further, with the lens 3, it is easy to control a spot size ofexcitation light which strikes the light-emitting section 4.

A laser beam that is emitted from a light-emitting point of a laser chip21 is light that has a spatial light intensity distribution in the formof a Gaussian profile. The laser beam is converted into light that has aspatial light intensity distribution in the form of a top-hat profile bybeing repeatedly reflected inside the first optical member 29.

The exit end 29 b is positioned inside a cap 25. The laser beam whichhas come out through the exit end 29 b travels to the outside of thelaser element 40 through a cap glass 30 provided to the cap 25.

Between the exit end 29 b and the cap glass 30, an AR coating film 24 ais provided. By providing the AR coating film 24 a, it is possible toprevent a reduction in efficiency of emission, which reduction is due toa laser beam reflected at a surface of the cap glass 30 toward inside ofthe first optical member 29.

<Effects of Laser Element 40>

According to the laser element 40, since a laser beam emitted from thelaser chip 21 passes through the first optical member 29, it is possibleto convert the spatial light intensity distribution of the laser beaminto a top-hat distribution.

As a result, it is possible to irradiate the laser beam irradiationsurface 4 a with a laser beam having an almost uniform light intensity,and thus possible to prevent the efficiency of conversion by a phosphorfrom decreasing due to heat of the laser beam.

Further, it is possible to transmit the laser beam with little leakageof the laser beam from the first optical member 29. This makes itpossible to increase energy efficiency.

Further, since a tolerance of optical coupling between the first opticalmember 29 and the laser chip 21 is very large, the laser element 40 isadvantageous in that it does not require precise alignment and is alsoresistant to vibration.

Embodiment 4

The following description discusses still a further embodiment of thepresent invention with reference to FIG. 8. Note that members which arethe same as those in Embodiments 1 to 3 are assigned identical referencenumerals, and their descriptions are omitted here.

According to the present embodiment, the headlamp 1 includes a laserelement 50 instead of the laser element 2. The other configurations arethe same as those of Embodiments 1 to 3.

<Configuration of Laser Element 50>

FIG. 8 is a view schematically illustrating a configuration of the laserelement 50 applicable to the headlamp 1. The laser element 50 includes amultimode fiber (a first optical member) 31 instead of the optical rod23. The multimode fiber 31 is an optical fiber in which the number ofmodes of light propagating therethrough is two or more, and has adouble-layer structure which is constituted by (i) a core and (ii) acladding which is lower in refractive index than the core and enclosesthe core. The core is mainly made of, for example, silica glass (silicondioxide) which causes little loss of laser beam absorption. The claddingis mainly made of, for example, silica glass or a synthetic resinmaterial which is lower in refractive index than the core.

The length of the multimode fiber 31 is, for example, 20 mm, and thediameter of its end is, for example, 0.4 mm. However, the length and thediameter of the multimode fiber 31 are not limited to the above.

A laser beam enters the multimode fiber 31 through an entrance surface31 a at one end of the multimode fiber 31. The laser beam which hasentered the multimode fiber 31 travels while being reflected inside thecore of the multimode fiber 31, and then goes out through an exitsurface 31 b at the other end.

The laser beam which has come out through the exit surface 31 b iscontrolled by a lens 3. By providing the lens 3, it is possible to causethe exit surface 31 b of the multimode fiber 31 and a spot of excitationlight incident on a light-emitting section 4 to be optically conjugatewith each other, and thus possible to cause the laser beam, which hascome out through the exit surfaced 31 b, to be efficiently directed to alaser beam irradiation surface 4 a of the light-emitting section 4.Further, by providing the lens 3, it is possible to easily control aspot size of the excitation light which strikes the light-emittingsection 4.

A laser beam emitted from a light-emitting point of a laser chip 21 islight which has a spatial light intensity distribution in the form of aGaussian profile The laser beam is converted into light having a spatiallight intensity distribution in the form of a top-hat profile by beingrepeatedly reflected inside the core of the multimode fiber 31.

Note that, an AR coating film 24 can be provided on the entrance surface31 a of the multimode fiber 31 in the same manner as the laser element2.

<Effects of laser element 50>

According to the laser element 50, since a laser beam emitted from thelaser chip 21 passes through the multimode fiber 31, it is possible toconvert the spatial light intensity distribution of the laser beam intoa top-hat distribution.

As a result, it is possible to irradiate the laser beam irradiationsurface 4 a with a laser beam having an almost uniform light intensity,and thus possible to prevent the efficiency of conversion by a phosphorfrom decreasing due to heat of the laser beam.

Further, with use of the multimode fiber 31, it is easier to form alonger optical member (for example, 1 m to 5 m) than when using theoptical rod 23 or the first optical member 29 (usually, approximately 10mm to 20 mm long). In a case where an optical member is long, a laserbeam is reflected a larger number of times inside the optical member,and thus light intensity distribution becomes uniform. This makes itpossible to achieve a spatial light intensity distribution in the formof a top-hat profile which is more uniform in distribution. Furthermore,the long optical member is advantageous also in that it is possible todetermine the position of the laser element more freely.

Embodiment 5

The following description discusses still yet a further embodiment ofthe present invention with reference with FIG. 9. Note that memberswhich are the same as those in Embodiments 1 to 4 are assigned identicalreference numerals, and their descriptions are omitted here.

According to the present embodiment, the headlamp 1 includes a laserelement 60 instead of the laser element 2. The other configurations arethe same as those of Embodiments 1 to 4.

<Configuration of Laser Element 60>

FIG. 9 is a view schematically illustrating the laser element 60applicable to the headlamp 1. The laser element 60 includes a first lens(a first optical member) 32 and a second lens (a first optical member)33, instead of the optical rod 23.

The first lens 32 concentrates a laser beam emitted from a laser chip 21to a principal point of the second lens 33, and the second lens 33directs the laser beam thus concentrated by the first lens 32 to a laserbeam irradiation surface 4 a of a light-emitting section 4. That is, thelaser element 60 is optically arranged such that (i) through the firstlens 32, an exit end surface of the laser chip 21 is conjugate with thesecond lens 33 and (ii) through the second lens 33, the first lens 32 isconjugate with the light-emitting section 4.

The first lens 32 is fixed to a cap 25 with a fixing part 34 made oflow-melting glass. The second lens 33 is fixed to the cap 25 with afixing part 35 made of low-melting glass. Note, however, that how to fixthe first lens 32 and the second lens 33 is not limited to the above,and therefore the first lens 32 and the second lens 33 can be fixed inany manner.

The first lens 32 and the second lens 33 can be a pair of convex lenses.

The number of lenses sealed in the laser element 60 is not limited totwo, and can be three or more.

Further, the second lens 33 can be provided outside the laser element60.

Specifically, a part of a plurality of lenses which constitute a firstoptical member can be provided inside the laser element 60, and theother part of the plurality of lenses can be provided outside the laserelement 60.

According to a configuration in which the first lens 32 and the secondlens 33 are provided, a lens 3 does not necessarily have to be provided.Whether or not to provide a lens 3 can be determined on the basis of thecharacteristics and/or arrangement of the first lens 32 and the secondlens 33.

Further, the second lens 33 can be the same in size as the first lens32, and also can be larger in size than the first lens 32.

<Effects of Laser Element 60>

According to the laser element 60, it is possible to convert the spatiallight intensity distribution of a laser beam emitted from the laser chip21 into a top-hat distribution by the first lens 32 and the second lens33.

As a result, it is possible to irradiate the laser beam irradiationsurface 4 a with a laser beam that has an almost uniform lightintensity, and possible to prevent the efficiency of conversion by aphosphor from decreasing due to heat of the laser beam. Further, byusing the first lens 32 and the second lens 33 as a first opticalmember, it is possible to realize a laser element and the headlamp 1 atlower costs than a case of using the optical rod 23 or the like.

A light-emitting device in accordance with an embodiment of the presentinvention includes a light-emitting section which emits fluorescenceupon receiving excitation light from an excitation light source, energyintensity distribution of the excitation light, which is received by thelight-emitting section, being a top-hat distribution.

According to the above configuration, the energy intensity distributionof the excitation light received by the light-emitting section is thetop-hat distribution. The top-hat distribution is an almost-uniformenergy intensity distribution. Therefore, it is possible to prevent thelight-emitting section from being excited by excitation light that islocally high in energy intensity, and possible to reduce the possibilityof temperature quenching due to locally-high temperature. As a result,it is possible to increase the efficiency of conversion of theexcitation light into fluorescence.

Further, it is possible to increase a contrast between a part which isexcited and a part which is not excited, and possible to achieve asimple optical design.

Further, it is preferable that the light-emitting device furtherincludes a first optical member which converts the energy intensitydistribution of the excitation light into the top-hat distribution.

According to the above configuration, it is possible, in a case wherethe energy intensity distribution of the excitation light emitted fromthe excitation light source is for example a Gaussian distribution, toconvert the energy intensity distribution into the top-hat distributionby the first optical member.

Further, it is preferable that there is a space between the firstoptical member and the light-emitting section.

According to the above configuration, it is possible for thelight-emitting section to emit, upon receiving the excitation light fromthe first optical member, fluorescence to the side from which theexcitation light comes. Accordingly, it is possible to realize “areflective light-emitting section” (described later).

Further, it is preferable that the light-emitting device furtherincludes a projection member configured to project illumination light ina desired direction, which illumination light includes the fluorescenceemitted from the light-emitting section.

The projection member can be a reflector.

According to the above configuration, it is possible to project, in adesired direction, illumination light which includes fluorescenceemitted from the light-emitting section. Further, the light-emittingsection, which is excited by excitation light having an energy intensitydistribution in the form of a top-hat profile, emits fluorescence havingan energy intensity distribution in the form of a top-hat profile. Whenthe projection member projects such fluorescence, almost all beams ofthe fluorescence emitted from the light-emitting section can be used.This makes it possible to realize a highly efficient projection system.

Note that, in a case where the light-emitting section is excited byexcitation light having a Gaussian distribution, part of thelight-emitting section, which part is excited by a portion of theexcitation light which portion corresponds to the edge portion of theGaussian distribution, cannot emit fluorescence having a sufficientluminous flux. The fluorescence emitted from the portion correspondingto the edge portion becomes stray light, which is not efficiently used.

Further, it is preferable that the light-emitting device furtherincludes a second optical member which (i) controls a spot size of theexcitation light which has come out of the first optical member and (ii)directs the excitation light to the light-emitting section.

According to the above configuration, it is possible to control a spotsize of the excitation light which is received by the light-emittingsection, and possible to change a range of irradiation with theillumination light.

Further, it is preferable that the first optical member has an exitsurface through which the excitation light goes out; and the exitsurface is polygonal.

According to the above configuration, since a shape of the exit surfaceof the optical member is polygonal (for example, rectangle), it ispossible, in a case of arranging a plurality of optical members or aplurality of spots of excitation light so that they are in contact witheach other, to reduce a space between the plurality of optical membersor between the plurality of spots of the excitation light.

Further, it is preferable that the first optical member has an exitsurface through which the excitation light goes out; and the exitsurface has a shape that corresponds to a desired light distributionpattern.

According to the above configuration, since the exit surface has a shapewhich corresponds to a desired light distribution pattern, it ispossible to cause a light distribution pattern of the illumination lightto be the desired light distribution pattern.

The first optical member can include a multimode fiber, a hollow memberwhose inner surface serves as a reflection surface, or an optical rod.

According to the above configuration, it is possible, in a case wherethe energy intensity distribution of excitation light emitted from theexcitation light source is for example a Gaussian distribution, toconvert the energy intensity distribution into the top-hat distributionby the first optical member.

Further, it is preferable that the first optical member includes a firstlens and a second lens; through the first lens, the excitation lightsource is conjugate with the second lens; and through the second lens,the first lens is conjugate with the light-emitting section.

According to the above configuration, it is possible to easily control aspot size of excitation light that is received by the light-emittingsection.

Further, it is preferable that the first optical member has an entrancesurface which receives the excitation light; and the entrance surface isprovided with an antireflection structure.

According to the above configuration, since the antireflection structure(for example, antireflection film (AR coating)) is provided on theentrance surface of the optical member, the excitation light isprevented from being reflected by the entrance surface. As a result, itis possible to reduce losses of the excitation light which losses wouldoccur when the excitation light enters the optical member.

Further, it is preferable that a most intensive part of a distributionof the fluorescence, which is emitted from the light-emitting section,is included in a space where there is a surface of the light-emittingsection which surface receives the excitation light, the space being oneof two spaces separated by a plane including a surface on which thelight-emitting section is provided.

According to the above configuration, fluorescence is emitted to theside from which the excitation light comes (this structure is referredto as a “reflective light-emitting section” in the subject application).According to a structure in which fluorescence is emitted mainly to aside opposite to a side on which there is a surface of thelight-emitting section which surface is irradiated with the excitationlight (this structure is referred to as a “transmissive light-emittingsection”), energy intensity distribution spreads when the excitationlight passes through the light-emitting section, and thus heat is alsodispersed. In contrast, the reflective light-emitting section does nothave such a disadvantage. Therefore, using the optical member in areflective light-emitting section is especially useful.

Further, a vehicle headlamp including the light-emitting device and anillumination device including the light-emitting device are encompassedin the technical scope of the present invention.

Further, it is preferable that the light-emitting device furtherincludes a first optical member which converts the energy intensitydistribution of the excitation light into the top-hat distribution; andthe first optical member has an exit surface through which theexcitation light goes out, and the exit surface has a shape thatcorresponds to a light distribution pattern of a low beam to be emittedfrom the vehicle headlamp.

According to the above configuration, it is possible to cause a lightdistribution pattern of a vehicle headlamp to comply with the lightdistribution pattern of a low beam designated by law.

ADDITIONAL REMARKS

The present invention is not limited to the description of theembodiments above, but may be altered by a skilled person within thescope of the claims. An embodiment based on a proper combination oftechnical means disclosed in different embodiments is encompassed in thetechnical scope of the present invention.

For example, a fly-eye lens can be used to achieve a laser beam having atop-hat distribution. A fly-eye lens is a lens which is made up of aplurality of lenses (double-convex lenses or semi convex lenses)arranged in a matrix manner.

INDUSTRIAL APPLICABILITY

The present invention is applicable to a light-emitting device and anillumination apparatus. The present invention is particularly applicableto a vehicle headlamp, and the like.

REFERENCE SIGNS LIST

-   -   1 Headlamp    -   2 Laser element (Excitation light source)    -   3 Lens (Second optical member)    -   4 Light-emitting section    -   20 Laser element (Excitation light source)    -   21 Laser chip (Excitation light source)    -   23 Optical Rod (First optical member)    -   23 a Entrance surface    -   23 b Exit surface    -   24 AR coating film (Antireflection structure)    -   29 First optical member    -   29 a Entrance end    -   29 b Exit end    -   29 c Inner surface    -   24 a AR coating film (Antireflection film)    -   31 Multimode fiber (First optical member)    -   31 a Entrance surface    -   31 b Exit surface    -   32 First lens (First optical member)    -   33 Second lens (First optical member)    -   40 Laser element (Excitation light source)    -   41 Laser beam spot    -   50 Laser element (Excitation light source)    -   60 Laser element (Excitation light source)

1. (canceled)
 2. A light-emitting device, comprising: a first opticalmember which transmits excitation light emitted from an excitation lightsource and has an exit surface through which the excitation light goesout; a light-emitting section which emits fluorescence upon receivingthe excitation light from the exit surface; and a second optical memberwhich is provided between the exit surface and the light-emittingsection, the exit surface and a spot of the excitation light on a lightirradiation surface of the light-emitting section being opticallyconjugate with each other, and a distribution profile of the excitationlight on the exit surface being identical with a distribution profile ofthe excitation light on the light-emitting section.
 3. Thelight-emitting device of claim 2, wherein: the first optical member isan optical fiber having one exit surface.
 4. The light-emitting deviceof claim 2, wherein: a spatial light intensity distribution ofexcitation light on the exit surface of the first optical member has atop-hat profile.
 5. The light-emitting device of claim 2, wherein: thefirst optical member has a structure inside in which the excitationlight passing therethrough is reflected multiple times.
 6. Thelight-emitting device of claim 2, wherein: a region of the exit surfacethrough which the excitation light passes has at least one linear edge.7. The light-emitting device of claim 2, wherein: the exit surface is aplanar surface.
 8. The light-emitting device of claim 2, wherein: thefirst optical member is a multimode fiber or an optical rod.
 9. Anillumination device comprising a light-emitting device of claim
 2. 10. Avehicle headlamp comprising an illumination device of claim 9.